1. Field of the Invention
The present invention relates to a color contact image sensor comprising an array of sensor elements whose length is equal to the width of the subject to be read, for reading the subject while being held in substantial contact with the subject.
2. Description of the Related Art
Contact image sensors have a length equal to the width of a subject to be read thereby, and operate to read the subject while being held in substantial contact with the subject. Therefore, the contact image sensors are advantageous in that they have an optical path shorter than that of CCDs (charge-coupled devices) and make a reading device small in size through the contact image sensors themselves are relatively large.
Contact image sensors are roughly classified into the following two categories:
In one of the categories, a focusing optical system such as a rod lens array or the like is positioned between an image sensor and a subject for focusing the life-size image of the subject on the sensor surface. Although this type is called a contact image sensor, the image sensor and the focusing optical system need to be spaced from each other and the focusing optical system and the subject also need to be spaced from each other for focusing the image of the subject onto the image sensor. Usually, it is necessary for the image sensor and the subject to be spaced from each other by a distance of 10 mm or greater.
In the other category, a thin glass plate which is about 50 .mu.m thick is attached to the surface of an image sensor, which reads a subject that is held in contact with the glass plate. This type is referred to as a full contact image sensor. While the full contact image sensor doe s not have a focusing optical system, the resolution will greatly be reduced if the subject becomes spaced from the image sensor. Another problem of the full contact image sensor is that the sensor elements produce shadows that make it difficult to illuminate the subject.
To solve the above problems of the full contact image sensor, there has been proposed a full contact image sensor having sensor elements with light guide apertures defined therein and a light guide disposed between the sensor elements and a subject and comprising an array of optical fibers, as disclosed in Japanese laid-open patent publication No. 291935/94.
The disclosed full contact image sensor will be described below with reference to FIG. 1 of the accompanying drawings. As shown in FIG. 1, the full contact image sensor has an LED(light emitting diode) array 102 for emitting illuminating light 104a, a sensor board 110 having an array of sensor elements for photoelectrically converting incident light into electric charges depending on the intensity of the incident light, and a light guide 112 bonded to the sensor board 110 by a transparent adhesive layer 108.
The sensor board 110 comprises a glass substrate 115, an opaque common electrode 116 mounted on the glass substrate 115, a photosensitive layer 117 mounted on the common electrode 116, and a transparent individual electrode 118 mounted on the photosensitive layer 117, jointly making up photodiodes as the sensor elements. The common electrode 116, the photosensitive layer 117, and the individual electrode 118 have a plurality of light guide apertures 107 defined therein. The light guide 112 comprises a plurality of optical fibers 112a each covered with a light absorbing layer 112b, and is bonded to the surface of the individual electrode 118 by the adhesive layer 108. The LED array 102 is disposed on one side of the sensor board 110 near the glass substrate 115.
The illuminating light 104a emitted from the LED array 102 passes through the light guide apertures 107 in the sensor board 110 into the optical fibers 112a, illuminating a subject 105. The light which has illuminated the subject 105 is reflected as reflected light 104b by the subject 105, and the reflected light 104b travels back through the optical fibers 112a and is applied to the photo sensitive layer 117. The photo sensitive layer 117 generates electric charges depending on the intensity of light applied thereto, and stores the generated electric charges. The stored electric charges are then detected at a given time for thereby reading the image of the subject 105.
Image sensors may read a colored subject through color filters placed on their surfaces. Conventional color image sensors have blue (B), green (G), and red (R) color filters associated with respective pixels or color columns. The color filters may be produced by a pigment dispersing process, a dyeing process, a printing process, or the like.
In the contact image sensor disclosed in Japanese laid-open patent publication No. 291935/94, the photodiodes formed by the photosensitive layer and the electrodes generate electric charges are commensurate with the intensity of the light reflected by the subject. Therefore, the disclosed contact image sensor can only detect gray levels, and hence is unable to detect the colors of colored subjects.
One solution will be to attach color filters to the surface of the image sensor. However, if color filters associated with at least respective pixels are combined with a contact image sensor which has light guide apertures in the sensor elements, then the utilization of light by the contact image sensor is lowered, resulting in a poor S/N ratio.
A conventional arrangement in which color filters are combined with a contact image sensor having light guide apertures will be described below.
FIG. 2 of the accompanying drawings shows in cross section the manner in which a subject is being read by a pixel of a contact image sensor having light guide apertures and combined with a color filter. As shown in FIG. 2, illuminating light 154a emitted from a light source 152 passes through a light guide aperture 157, a color filter 169, and an optical fiber 162a and then illuminates a subject 155. The light is then reflected by the subject 155, passes through the optical fiber 162a and the color filter 169, and reaches a photosensitive layer 167. Therefore, the light from the light source 152 passes the color filter 169 twice until it reaches the photosensitive layer 167 after being reflected by the subject 155.
FIG. 3 of the accompanying drawings shows the transmittance of blue (B), green (G), and red (R) color filters that are manufactured by the pigment dispersing process, after the light has passed through the color filters once. FIG. 4 of the accompanying drawings shows the transmittance thereof after the light has passed through the color filters twice. A review of FIGS. 3 and 4 indicates that the utilization of light by the contact image sensor is lowered when the light passes through the color filters twice. Specifically, the utilization of blue (B) light is reduced to 58%, the utilization of green (G) light is reduced to 49%, and the utilization of red (R) light is reduced to 82%. This is because a loss of light occurs each time the light is reflected by interfaces of the color filters and absorbed by the binder of the color filters. While the absorption of light within the color filters can be reduced to some extent by making the color filters thinner, the reflection of light by the interfaces of the color filters is unavoidable. Accordingly, when the light passes the color filters twice, the utilization of light by the contact image sensor is lowered, resulting in a reduction in the level of signals that are generated by the contact image sensor. The reflection of light by the interfaces of the color filters is responsible for the occurrence of stray light, which tends to cause a reduction in the resolution.